Air Brake System Components and Functions
Florida’s CDL air-brake test asks you to explain how a compressed-air system makes, stores, cleans, and delivers pressure to the foundation brakes. You need to know what each component does and how a failure changes the system’s behavior. Know the order of air flow and the job of each protective device.
Production and storage
- The engine-driven compressor produces air; it does not store it.
- The governor cycles the compressor: cut-out near 125 psi, cut-in near 85 psi on typical systems.
- The air dryer removes moisture and oil vapor before air reaches the reservoirs.
- The wet tank receives air first; check valves keep air from flowing backward between tanks.
- A safety valve, usually set near 150 psi, protects the system if the governor fails.
- Drain cocks at each reservoir let you remove moisture during inspection.
Control and application
The foot valve (treadle valve) meters air to the brakes. On long vehicles, relay valves speed application and release by sending a large volume of air to the chambers. Push rods transfer chamber force to slack adjusters, then to the S-cam and brake shoes. The slack adjuster converts push-rod travel into rotation of the S-cam. Spring brakes are mechanically applied by springs and released by air, so they are parking/emergency brakes, not the service brakes.
Worked example
A driver notices the dash gauge climbing past 130 psi and hears a loud pop from the wet tank. The governor has likely failed to cut out the compressor. The safety valve opens near 150 psi to limit pressure. The correct response is to stop safely and have the system repaired; the safety valve is a last-resort protector, not a normal pressure regulator.
Traps that cost marks
- Saying the governor “makes” air; it only controls the compressor.
- Confusing the safety valve with the low-pressure warning device.
- Treating spring brakes as service brakes.
- Forgetting check valves and the wet tank when tracing air flow.